Hip-joint or knee-joint replacement is a widely used orthopedic procedure to correct various types of joint injury, such as those caused by osteoarthritis, fracture dislocations, rheumatic arthritis, and aseptic bone necrosis. A variety of prosthetic devices for hip and knee-joint replacement are known. The usual device used in femoral-side hip-joint replacement includes an elongate stem having a ball-like joint implant formed at one end. The stem is constructed for insertion into the medullary region of a femur adjacent the joint, and may be secured in the medullary region by a grout material, such as methylmethacrylate. In time, the bone reacts to the grout material or smooth metal stem by forming a soft-tissue lining around the stem, and this lining mediates load transfer from the prosthetic device to the bone.
This type of prosthetic device has not been entirely satisfactory, particularly in active younger patients, where it is important to form a stable, long-lasting prosthetic attachment. The soft-tissue lining that forms about the device tends to loosen over time, particularly with continued shear loads, i.e., loads applied substantially in the direction of the axially extending bone/stem interface, and the loosening may become great enough in time to require surgical revision. Also, the relatively low tolerance of force transfer per unit area of interface requires a large bone/stem interface, which, in younger patients, may exceed the available bone stock.
The problems associated with anchorage via soft tissue along a prosthesis stem have been overcome partially by using a prosthesis whose stem surface allows direct attachment without an interposed soft tissue layer. Such surfaces include micropore surfaces that allow attachment via ingrowth and/or attachment of bone, and ceramic surfaces that allow actual bonding of bone. Following surgical implantation of the stem, the surrounding bone tissue gradually forms a biological fixation matrix with the stem surface by tissue growth into or onto the surface. Because of the stronger interface between the bone and the stem, which allows a relatively large force per unit area without loosening, problems of loosening and detachment are largely avoided and the force transfer area can be made smaller.
A limitation of the biological-fixation bonding approach, however, is the need to keep the prosthesis mechanically fixed with respect to the bone over a 2-3 month post-operative period, during which the biological fixation is occurring. If relative movement between the implant stem and bone is allowed to occur before biological fixation is complete, a fibrous tissue layer which acts to prevent good biological fixation develops at the interface and progression to gross loosening is likely.
Another limitation of prosthetic devices which rely on biological fixation, particularly fixation to an elongate stem within the intramedullary region of a bone, is the problem of stress protection of the bone region between the area of force application to the prosthesis and the area of load transfer to the bone. Stress protection is due to the rigid attachment between the prosthetic device and bone which occurs in biological fixation and to the relatively high elastic modulus of the implant material, which typically is 5-15 times greater than that of the surrounding bone. These two factors combine to transfer a stress from the area of stress loading on the implant through the more rigid implant, rather than through the surrounding bone tissue. For example, in a hip-joint prosthesis biologically anchored to the bone by an entire elongate stem, axial stress on the upper joint is transferred largely through the stem to the bone connection farthest from the joint, rather than through the intermediate bone region surrounding the part of the stem closest to the joint. As a result, the intermediate bone region tends to be resorbed over time due to a lack of deformation stressing. The gradual loss of bone support in the region of the stem increases the bending load that must be borne by the stem, and this can lead to implant fatigue and failure.
The problem of maintaining a motionless bone-prosthesis interface during the post-operative period when bony attachment is occurring may be partially solved by surgically fastening the prosthetic device to the bone structure by screws or the like. This method has been proposed for use in fastening a knee-joint prosthesis to a surgically formed, substantially planar surface of the bone. Typically, the prosthesis is attached by two or more screws, each tightened to hold the prosthesis against the bone surface with a selected compression. However, since the bone quickly accommodates to the applied force of the screws, by viscoelastic creep, the compression, and thus the resistance to the implant movement relative to the bone, is quickly lost. If interface movement does occur from a single episode of overloading, then any residual compression is permanently lost. More movements result in build-up of fibrous tissue, preempting biological bone fixation to the implant. Only with unphysiologic post-operative protection of the joint, resulting in joint stiffness and muscle wasting, and with demanding operative technique, can this risk of loosening be reduced to safe level. The device also suffers from problems of stress protection and non-physiological load transfer, inasmuch as loading force applied to the prosthesis is transferred directly through the screws, rather than through the region of bone through which the screws extend. This can lead to loss of bone integrity in the stress protected area.
Prosthetic devices having spring-loaded mechanisms for holding a joint-replacement prosthesis against a planar surface of the bone, to immobilize the prosthesis on the bone, have been proposed, e.g., in U.S. Pat. No. 4,129,903. Devices of this type solve some of the above-noted problems associated with prosthesis attachment to the bone, in that the prosthesis is held against the bone under relatively constant tension in the post-operative period, with or without provision for biological fixation. Nonetheless, limited movement may occur when the major loading stresses (in the principal direction of weight transfer on the joint) are not normal to the plane of the interface between the bone and prosthetic device and it is necessary to rely on a grouting compound to prevent shear motions. Further, such devices use a rigid stem or shaft for anchoring the implant to the bone, substantially stress protecting the bone traversed by the stem from physiologic shear, rocking, and/or axial rotation stresses.